Point the design-time factory at the stack the repo ships

`dotnet ef database update --project src/DodoSSH.Infrastructure` — the
command the README documents — failed on a clean machine. The design-time
default named `dodossh_design` as user `postgres` with no password, which is
a database this repository never creates, while the development compose
stack creates `dodossh`/`dodossh`. The failure arrives as a SCRAM
authentication error, so it reads like a broken container rather than a
stale default.

The default is now the compose stack, since that is the only local database
the repo defines. DODOSSH_DESIGN_CONNECTION still overrides it, and a real
deployment migrates through that or the migrator job.

Also documents running the thing end to end, which the README never covered:
the four commands in order, that migrations are a separate step because the
API deliberately fails readiness rather than migrating, and the three M1 gaps
visible in the first five minutes — so they are expected rather than
diagnosed.

src/DodoSSH.Infrastructure/DodoDbContextFactory.cs

@@ -17,14 +17,28 @@ public sealed class DodoDbContextFactory : IDesignTimeDbContextFactory<DodoDbCon
     /// <summary>Environment variable naming the database to work against.</summary>
     /// <remarks>
     /// Only <c>database update</c> and <c>dbcontext script</c> actually connect. Generating a migration
-    /// needs a provider but not a reachable server, which is why there is a usable default at all.
+    /// needs a provider but not a reachable server, which is why the default below is allowed to be a
+    /// guess at all.
     /// </remarks>
     public const string ConnectionVariable = "DODOSSH_DESIGN_CONNECTION";
 
+    /// <summary>
+    /// Where <c>dotnet ef</c> connects when nothing says otherwise.
+    /// </summary>
+    /// <remarks>
+    /// The development compose stack, because that is the only local database this repository defines. A
+    /// default naming a server nobody creates makes the documented <c>dotnet ef database update</c> fail on
+    /// every clean machine, which is a poor first impression of a self-hosted product — and the failure
+    /// arrives as a SCRAM authentication error, which reads like a broken stack rather than a stale
+    /// default. These credentials are the compose file's own; a real deployment migrates through
+    /// <see cref="ConnectionVariable"/> or the migrator job.
+    /// </remarks>
+    private const string DevelopmentStackConnection =
+        "Host=localhost;Port=5432;Database=dodossh;Username=dodossh;Password=dodossh";
+
     /// <inheritdoc />
     public DodoDbContext CreateDbContext(string[] args) =>
-        Create(Environment.GetEnvironmentVariable(ConnectionVariable)
-            ?? "Host=localhost;Database=dodossh_design;Username=postgres");
+        Create(Environment.GetEnvironmentVariable(ConnectionVariable) ?? DevelopmentStackConnection);
 
     /// <summary>
     /// Builds a context for one database, configured as the API configures its own.